1. Field of the Invention
The invention relates to an electrostatic latent image developer used for developing latent images formed by electrophotography, and an image-forming apparatus using the same.
2. Description of the Related Art
Methods for visualizing image information by forming latent images by electrophotography are presently used in various fields. In electrophotography, an electrostatic latent image is formed on the surface of a photoreceptor through electrically charging and exposure steps, and is developed (made visual) with an electrostatic latent image developer (hereinafter, simply referred to as “developer” in some cases) containing a toner to form a toner image, and the toner image is fixed on a recording medium through transfer and fixing steps.
Replacement of offset printers with electrophotographic printers which enable ultra-high speed, on-demand printing at a linear velocity exceeding 1,000 mm/second has been being considered in recent years to print newspaper and direct mail. In an effort to develop such electrophotographic printers, attempts are being made to improve the substantial printing volume by increasing the printing speed as well as adapting the electrophotographic printers to wide recording media. However, stress applied to a developer is proportional to the square of printing speed. Therefore, when printing speed is increased to a linear velocity as high as 1,000 mm/second or more, which corresponds to outputting about 400 A4-size sheets of paper or more for one minute, the developer suffers a stress far above the stress applied to the developer used in a desktop low speed printer.
An electrically conductive material, such as carbon black, used for adjusting electric resistance is usually contained in a coating resin on the surface of each carrier core in order to optimize printing performance. In the case of a color printer with a linear velocity as high as 1,000 mm/second or more, the electrically conductive material separates from the carrier alone or together with the coating resin due to a stress on the developer, and the free electrically conductive material stains color toners.
As for maintenance of printers, the interval for exchanging a developer in a high-speed printer is required to be almost the same as that in a low-speed printer. Therefore, the life of the developer used in a high-speed printer needs to be as long as that in a low-speed printer. Accordingly, it is necessary to prevent the electrically conductive material such as carbon black from separating from the coating resin of the carrier in a high-speed color printer and to provide the high-speed color printer with durability equal to or more than that of monochromatic printers.
In a proposed method for preventing carbon black from separating from the carrier to be mixed with a color toner, a magnetic core (core) of the carrier is coated with a coating agent containing carbon black, and then the coated magnetic core is further coated with the same coating agent containing no carbon black (see, for example Japanese Patent Application Laid-Open (JP-A) No. 8-179570). However, the surface coating layer abrades during a continuous operation of the high-speed printer, and staining of color toners by carbon black becomes evident as a result of the continuous operation, though no separation of carbon black is observed at the start of the operation.
On the other hand, as for fixing systems, the most crucial issue in the high-speed printer is to prevent paper from jamming and paper powder from occurring by friction between the printer and paper sheet. A non-contact fixing method that allows less contact with recording media and seldom causes paper jamming is desirable, and an open fixing method and a flash fixing method are usually effective for this purpose. A printer conducting an optical fixing method (referred to as a flash fixing method in some cases) is particularly attracting attention, since the printer provides high image quality and compatibility with a wide range of media, is able to start quickly without any standby power requirement, and has high reliability against paper jamming.
Therefore, stability of the developer is particularly important when employing the optical fixing method. Moreover, it is a crucial issue to stabilize characteristics of the developer in the ultra-high speed, on-demand printer.
Accordingly, there is a need for an electrostatic latent image developer having a bright color with a long life in developing an ultra-high speed electrophotographic printer. There is also a need for an image-forming apparatus using the electrostatic latent image developer.